Method for separating air by means of cryogenic distillation

ABSTRACT

In an air-distillation method, purified air is cooled in an exchange line and then sent to a distillation column of a system of columns, and oxygen- and nitrogen-rich fluids are extracted from a column of the system of columns only during the repressurization phase. A purified airflow, constituting between 3% and 20% of the air compressed in the compressor, is used to at least partially pressurize the adsorber completing the regeneration phase thereof, and the airflow compressed in the compressor during the adsorption phase is substantially equal to the airflow compressed in the compressor during the pressurization of the adsorber. A portion of the purified air is sent to a turbine where it is decompressed and then sent into the atmosphere an as to ensure that it is kept at least partially cold during the entire cycle, and the amount of decompressed airflow sent into the air during the pressurization of an adsorber is less than the amount sent into the air during the adsorption phase of the same adsorber.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a §371 of International PCT ApplicationPCT/FR2012/050587, filed Mar. 21, 2012, which claims the benefit of FR1152733, filed Mar. 31, 2011, both of which are herein incorporated byreference in their entireties.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method for separating air by distillation ofair, in particular intended for producing oxygen and/or nitrogen and/orargon, of the type wherein the air to be distilled is purifiedbeforehand by means of at least two adsorbers which each follow, offset,a cycle wherein succeed an adsorption phase, at a pressure of the cycle,and a regeneration phase ending with a pressurisation of the adsorber.

BACKGROUND

The pressures of which it is question here are absolute pressures.

In this type of installation, the distillation of the air, compressedbeforehand by a compression apparatus, is carried out cryogenictemperatures and therefore requires that the air be purified in order toremove from therein the constituents of which the solidificationtemperatures are higher than the distillation temperature of the air,i.e. primarily water and carbon dioxide. The main objective of thedistillation of the air is to provide, in liquid and/or gaseous form,oxygen and/or nitrogen and/or argon. This production generates thecoproduction of fluids with a low oxygen content, such as, for example,impure nitrogen, called residual nitrogen, and nitrogen of the highestpurity, in liquid or gaseous form.

The purification of the air to be distilled is commonly carried out byadsorption of the disturbing constituents, by means in general of twobottles containing adsorbent substances arranged on a bed and operatingin alternating cycles. While one bottle is in adsorption phase, (i.e.,it is purifying the air that is to be distilled) the other bottle is inregeneration phase, (i.e., it is flushed with a dry regeneration gas,such as residual nitrogen) desorbing the impurities fixed on theadsorbent during its preceding adsorption phase. The regeneration of theadsorbent is increasingly effective when it is applied at a hightemperature and at a low pressure in relation to that maintained duringthe adsorption, which requires that a bottle terminating itsregeneration phase be pressurised, in order to restore a satisfactorycondition of pressure for its upcoming adsorption phase.

For this, the state of the art consists in sampling a fraction ofpurified air at the outlet of the bottle in adsorption phase and todecompress it to the bottle at the end of the regeneration phase, inorder to increase the pressure of the latter. During this operation, itis however indispensable to maintain the air flow to be sent to thedistillation constant in order to prevent any fluctuation in the supplyof the distillation apparatus and in order to maintain the production ofoxygen and/or nitrogen and/or argon. As such, during eachrepressurisation, the air compression apparatus must provide thissurplus of air which is used for the pressurisation. However, thisadditional air flow implies oversizing, and therefore an extra cost, ofthe compression apparatus. It is indeed asked to provide an additionalcompressed air flow of about 5% of the nominal air flow processed by thebottle in adsorption (according to the optimisation of the cycle),during a pressurisation duration of about 15 minutes for a bottle ofcommon size.

Subsequently, beyond the duration of pressurisation, the compressionapparatus operations with nominal air flow (i.e., that which correspondsto the separation capacity of the device for separating air).

As an example, for a maximum compressor flow of 100 kNm³/hr, the normalflow rate for use will be 95 kNm³/hr sent to the cold box wherein takesplace the distillation and 100 kNm³/hr solely for the singlepressurisation phase at the end of regeneration wherein 5 kNm³/hr of airis sent in order to pressurise one of the bottles.

Certain methods for separating air use a lost air system wherein all ofthe purified air is not sent to the distillation columns. In this casethere is generally an expansion turbine which will decompress to apressure close to the atmospheric pressure the excess air in relation tothe oxygen needs.

In this type of method, it is preferable to not apply the conventionalmethod of pressurisation of adsorbers.

SUMMARY OF THE INVENTION

The purpose of certain embodiments of the invention is to avoid theoversizing of compressors by reducing, even by eliminating, the increasein the air flow to be compressed in order to provide the additional gasrequired for the pressurisation of the bottles of adsorbent.

To this effect, an embodiment of the invention has for purpose a methodfor distilling air, in particular intended to produce oxygen and/ornitrogen and/or argon, of the type wherein the air to be distilled iscompressed beforehand in a compressor, purified by means of at least twoadsorbers which each follow, offset, a cycle wherein succeed anadsorption phase, at a high pressure of the cycle (P_(ads)) and aregeneration phase at a low pressure P_(atmos) ending with arepressurisation phase of the adsorber, the purified air is cooled in anexchange line and then sent to a distillation column of a system ofcolumns and oxygen-rich and nitrogen-rich fluids are withdrawn from acolumn of the system of columns, only during the repressurisation phasea purified air flow, constituting between 3 and 20% of the aircompressed in the compressor, is used to pressurise, at least partially,the adsorber completing its regeneration phase and the air flowcompressed in the compressor during the adsorption phase issubstantially equal to the air flow compressed in the compressor duringthe pressurisation of the adsorber, characterised in that a portion ofthe purified air is sent to a turbine wherein it is expanded and thensent to the atmosphere so as to provide at least partially therefrigeration requirements during the entire cycle and in that theexpanded air flow sent to the air during the pressurisation of anadsorber is lower than that sent to the air during the adsorption phaseof the same adsorber, even during the rest of the cycle beyond thepressurisation phase.

The term “substantially equal” covers the case wherein the air flowcompressed in the compressor during the adsorption phase differs by atmost 5%, more preferably by at most 3%, from the air flow compressed inthe compressor during the pressurisation of the adsorber. The two flowsare more preferably strictly equal.

According to other characteristics of this method, taken individually oraccording to the technically permissible combinations:

-   -   the air flow compressed in the compressor during the adsorption        phase of an adsorber is equal to the air flow compressed in the        compressor during the pressurisation of the adsorber;    -   the reduction of the air flow sent to the turbine and then to        the air during the repressurisation is equal to the air flow        used during the repressurisation in order to pressurise the        adsorber completing its repressurisation phase;    -   the quantity of air sent to the distillation is constant during        the entire cycle;    -   the reduction in the air flow sent to the turbine and then to        the air during the pressurisation of an adsorber is lower than        the air flow used during the repressurisation in order to        pressurise the adsorber completing its repressurisation phase;    -   during the repressurisation phase the air flow compressed in the        compressor increases in relation to the flow sent during the        rest of the cycle and the quantity of air sent to the        distillation remains equal to that sent during the rest of the        cycle;    -   a liquid flow is produced as a final product;    -   said method of purification is an adsorption of the PSA, TSA or        TPSA type;    -   air is expanded in a turbine and sent to a column of the system        of columns:    -   the system of columns is constituted by a double column        comprising a medium-pressure column and a low-pressure column    -   an oxygen-rich flow is withdrawn from the low-pressure column        and it is vaporised in the exchange line.

The term “PSA” used in this document means “Pressure swing adsorption”.The term “TSA” used in this document means “Temperature swingadsorption”. The term “TPSA” used in this document means “Temperatureand pressure swing adsorption”.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, claims, and accompanying drawings. It is to be noted,however, that the drawings illustrate only several embodiments of theinvention and are therefore not to be considered limiting of theinvention's scope as it can admit to other equally effectiveembodiments.

FIG. 1 shows a diagrammatical view of an installation for a methodaccording to an embodiment of the invention an embodiment of the presentinvention.

DETAILED DESCRIPTION

The invention shall be better understood when reading the followingdescription, provided solely by way of example and in reference to theannexed drawing, wherein:

-   -   FIG. 1 is a diagrammatical view of an installation for operating        the method according to the invention.

FIG. 1 shows an installation 1 for the distillation of air according tothe invention. This installation is for example intended to producegaseous oxygen OG, as well as liquid oxygen OL.

The installation 1 substantially comprises:

-   -   an air compressor 4;    -   an apparatus 6 for purifying air via adsorption, said apparatus        comprises, on the one hand, two adsorbers 7A, 7B in the form of        two bottles each containing adsorbent materials, for example        molecular sieve possibly with alumina, capable of adsorbing the        water and the carbon dioxide present in the air, and, on the        other hand, ducts and connection valves of which the arrangement        shall appear clearly during the description of the method        implemented in the installation 1 and which makes it possible to        successively submit each adsorber 7A, 7B to the air flow to be        distilled and to a regeneration gas of the adsorbent;    -   a lost air turbine 27;    -   a cold compressor 3;    -   a Claude turbine 5 sending air to the medium-pressure column;    -   a main thermal exchange line 8;    -   an apparatus for distilling air in the form of a double column        10 comprising a medium-pressure column 12, a low-pressure column        14 and a vaporiser-condenser 16 coupling these two columns, as        well as an argon separation column 26; and    -   a reservoir 18 for storing liquid oxygen.

The operation of the installation 1 of FIG. 1 is as follows.

The air to be distilled, compressed beforehand by the compressor 4, ispurified by one of the adsorbers 7A, 7B of the apparatus 6, then cooledby the main thermal exchange line 8 to an intermediate temperature. Theadsorption can be of the TSA, PSA or TPSA type. A portion 25 of the airis sent to a lost air turbine 27 and the expanded air is sent to theatmosphere after reheating in the exchanger 8. The rest of the aircontinues to be cooled. Another portion 29 of the air is sent to thecold compressor 3, sent back to the exchange line 8. A portion of thesupercharged flow is expanded in a turbine 5 to the medium pressure inorder to form the expanded flow 7. The expanded flow 7 in the vicinityof its dew point is introduced into the tank of the medium-pressurecolumn 12. The rest of the supercharged air 9 continues to be cooled inthe exchange line 8, is expanded in a valve V then is sent to anintermediate level of the medium-pressure column 12.

The vaporiser-condenser 16 vaporises liquid oxygen, for example having apurity of 99.5%, of the tank of the low-pressure column 14, bycondensation of gaseous nitrogen at the head of the medium-pressurecolumn 12.

“Rich liquid” LR (oxygen-rich air), sampled in the tank of themedium-pressure column 12, is injected, after expansion, at anintermediate level of the low-pressure column 14, while liquid nitrogenNL, substantially pure, is sampled at the head of the medium-pressurecolumn 12 in order to supply the reservoir 22 and the head of thelow-pressure column 14. Liquid nitrogen and/or liquid oxygen is producedas a final product, sent to the client in liquid form.

Impure or “residual” nitrogen NR, withdrawn from the top of thelow-pressure column 14, is sent back to the main thermal exchange line8, where it causes the cooling of the air to be distilled.

Liquid oxygen OL is withdrawn from the tank of the low-pressure column14 and supplies the storage reservoir 18. After pressurisation in thepump P, it is vaporised in the main thermal exchange line 8 anddistributed by a production pipe 32 in order to form pressurised gaseousoxygen.

An argon production column 26 is supplied from the low-pressure column14.

The operation of the installation that has just been described can beimplemented continuously, except for the operation of the purificationapparatus 6, which follows over time a pressure cycle of FIG. 2.However, it is possible for all of the fluids to not be producedconstantly, according to the needs of the client, the cost ofelectricity, etc.

The cycle of FIG. 2, of which the period is, by way of example, equal toabout 360 minutes for an adsorption pressure substantially equal to 20bars, comprises 4 successive steps I to IV. These four steps shall nowbe described successively for the adsorber 7A, with the understandingthat the adsorber 7B follows these same steps with a time delay ofsubstantially

$\frac{T}{2},$

by means of open or closed connection valves designated by the sameupcoming references as those of the adsorber 7A, with the letter A to bereplaced with the letter B and the state of each valve (open/closed) tobe inverted (closed/open).

During the step 1 (i.e., t=0

$\left. {t = \frac{T}{2}} \right),$

to the adsorber 7A is in adsorption phase under a high operatingpressure noted as P_(ads), while the adsorber 7B is in regenerationphase. The air compressed by the compressor 4 supplies the adsorber 7A,via an open valve 40A. The outlet of the adsorber 7A is connected to theexchange line 8, via an open valve 42A.

During the steps II, III and IV, the adsorber 7A is in regenerationphase, while the adsorber 7B is in adsorption phase. More precisely,during the step II, a valve 44A for venting the adsorber 7A to the airis open in such a way that the pressure inside the bottle of theadsorber 7A is brought to a pressure substantially equal to theatmospheric pressure, noted as P_(atmos) in FIG. 2.

During the step III, the valve 44A remains open and residual nitrogen NRwithdrawn at the head of the low-pressure column 14 then heated in theexchanger 8 supplies, via an open valve 46A, the adsorber 7A in order tocirculate therein against the current. This is the effective phase ofthe regeneration during which the impurities are desorbed and the bedsare regenerated. During the step IV, the valves 44A and 46A are closed,in order to allow for the pressurisation of the adsorber. In a firststep, i.e. during a first sub-step IV′, the pressurisation of theadsorber is provided by a purified air flow, via the open valve 42A,this purified air flow coming from the bottles 7A, 7B. The sub-step IV′is continued by the sub-step IV″ until the pressure inside the adsorber7A is substantially equal to the high pressure P_(ads), by opening thevalve 50.

By the method according to the invention, the pressurisation of eachadsorber no longer requires, during the step IV, to increase the flow ofthe compressor 4. In this way, the compressor 4 is sized in an optimummanner, i.e. in such a way that its nominal flow is substantiallyconstant. The investment and operating costs for this compressionapparatus are reduced, in relation to those of installations concerningprior art.

During the adsorption phase, the compressor 4 compresses 100 kNm³/hr ofair and all of the purified air is sent to the exchange line 8. 30kNm³/hr of air is sent to the lost air turbine 5. 70 kNm³/hr of air issent to the system of distillation columns.

During the pressurisation phase at the end of the regeneration phase,the compressor 4 compresses 100 kNm3/h of air, 95 kNm³/hr is sent to theexchange line 8 and 5 kNm³/hr is sent in order to pressurise anadsorption bottle. 25 kNm³/hr of air (therefore 5 kNm³/hr less) is sentto the lost air turbine 5 and 70 kNm³/hr of air is still sent to thesystem of distillation columns.

It shall be understood that this invention applies to any methodinvolving a lost air turbine, whether there is compression in a coldcompressor or not, a double column or not, a production of argon or not,pressurisation and vaporisation of liquid oxygen or not.

It shall also be understood that if the reduction in the lost air flowis less than the flow sent to the pressurisation, either the compressedflow will have to increase during the pressurisation and the distilledair flow remains unchanged or less air will be sent to the distillationand the compressed flow will remain unchanged.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims. The presentinvention may suitably comprise, consist or consist essentially of theelements disclosed and may be practiced in the absence of an element notdisclosed. Furthermore, if there is language referring to order, such asfirst and second, it should be understood in an exemplary sense and notin a limiting sense. For example, it can be recognized by those skilledin the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unlessthe context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means thesubsequently identified claim elements are a nonexclusive listing (i.e.,anything else may be additionally included and remain within the scopeof “comprising”). “Comprising” as used herein may be replaced by themore limited transitional terms “consisting essentially of” and“consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, makingavailable, or preparing something. The step may be performed by anyactor in the absence of express language in the claim to the contrary arange is expressed, it is to be understood that another embodiment isfrom the one.

Optional or optionally means that the subsequently described event orcircumstances may or may not occur. The description includes instanceswhere the event or circumstance occurs and instances where it does notoccur.

Ranges may be expressed herein as from about one particular value,and/or to about another particular value. When such particular valueand/or to the other particular value, along with all combinations withinsaid range.

All references identified herein are each hereby incorporated byreference into this application in their entireties, as well as for thespecific information for which each is cited.

1-13. (canceled)
 14. A method for distilling air to produce an air gasselected from the group consisting of oxygen, nitrogen, argon, andcombinations thereof, the method comprising the steps of: compressingthe air to be distilled in a compressor; purifying the air followingcompression using at least two adsorbers to form a purified air, whereinwhich each adsorber follows, offset, a cycle wherein succeed anadsorption phase, at a high pressure of the cycle (P_(ads)), and aregeneration phase at a low pressure P_(atmos) ending with arepressurization phase of the adsorber; cooling the purified air in anexchange line and then sending the purified air to a distillation columnof a system of columns; withdrawing oxygen-rich and nitrogen-rich fluidsa from a column (14) of the system of columns; and sending a portion ofthe purified air to a turbine wherein the portion of the purified air isexpanded and then sent to the atmosphere so as to provide at leastpartially the refrigeration requirements during the entire cycle and inthat the flow of the expanded air sent to the atmosphere during thepressurization of the adsorber is lower than that sent to the atmosphereduring the adsorption phase of the same adsorber, wherein solely duringthe repressurization phase a purified air flow, constituting between 3and 20% of the air compressed in the compressor, is used to pressurize,at least partially, the adsorber completing its regeneration phase,wherein the air flow compressed in the compressor during the adsorptionphase is substantially equal to the air flow compressed in thecompressor during the pressurization of the adsorber.
 15. The method asclaimed in claim 14, wherein the expanded air flow sent to the airduring the pressurization of an adsorber is less than that sent to theair during the rest of the cycle beyond the pressurization phase. 16.The method as claimed in claim 14, wherein the air flow compressed inthe compressor during the adsorption phase of an adsorber is equal tothe air flow compressed in the compressor during the pressurization ofthe adsorber.
 17. The method as claimed in claim 14, wherein thereduction of the air flow sent to the turbine and then to the air duringthe repressurization is equal to the air flow used during therepressurization in order to pressurize the adsorber completing itsrepressurization phase.
 18. The method as claimed in claim 17, whereinthe quantity of air sent to the distillation is constant during theentire cycle.
 19. The method as claimed in claim 14, wherein thereduction in the air flow sent to the turbine and then to the air duringthe pressurization of an adsorber is less than the air flow used duringthe repressurization in order to pressurize the adsorber completing itsrepressurization phase.
 20. The method as claimed in claim 19, whereinduring the repressurization phase the air flow compressed in thecompressor increases in relation to the flow sent during the rest of thecycle and the quantity of air sent to the distillation remains equal tothat sent during the rest of the cycle.
 21. The method as claimed inclaim 14, wherein a liquid flow is produced as a final product.
 22. Themethod as claimed in claim 14, wherein the purification method is anadsorption of the PSA, TSA or TPSA type.
 23. The method as claimed inclaim 14, wherein the air is expanded in a turbine and sent to a columnof the system of columns.
 24. The method as claimed in claim 14, whereinthe expanded air flow sent to the air during the pressurization of anadsorber is less than that sent to the air during the rest of the cyclebeyond any pressurization phase.
 25. The method as claimed in claim 14,wherein the system of columns is constituted by a double columncomprising a medium-pressure column and a low-pressure column.
 26. Themethod as claimed in claim 25, wherein an oxygen-rich flow is withdrawnfrom the low-pressure column and it is vaporized in the exchange line.